Metabolomics of Chloroplasts Combined with Photosynthetic Properties Reveal Low-Temperature Stress Responses in Sugarcane (Saccharum officinarum L.)

Sugarcane (Saccharum officinarum L.) is a heat-originated plant and it is valuable to investigate the adaptive mechanism of photosynthesis to low temperature. Here, we explored the photosynthetic properties at leaf and chloroplast level in Guitang 11 (GT11), Guitang 28 (GT28), Guitang 21 (GT21), and Xintaitang 22 (ROC22), which with different cold tolerance, and mesophyll (M), and bundle sheath (BS) chloroplasts of cold-sensitive variety ROC22 for metabolomics analysis at molecular level. The findings revealed that at leaf level, the net photosynthetic rate (P_n) was significantly reduced by low temperature, with the largest decrease in ROC22, and P_n was affected through a combination of stomatal and non-stomatal. The maximum photochemical efficiency of PSII (F_v/F_m) and PSI (P_m), and electron transfer rate (ETR) of sugarcane leaves were greatly decreased under chilling stress. At chloroplast level, the F_v/F_m of M and BS chloroplasts decreased significantly after low temperature. Metabolomics results showed that due to cold stress, there were 279 and 181 up-regulated and 314 and 252 down-regulated of differential metabolites of M and BS chloroplasts in ROC22, respectively. KEGG pathway analysis showed that biosynthesis of unsaturated fatty acids, purine, porphyrin and chlorophyll metabolism were enhanced in M and BS chloroplasts after low temperature. The xanthophyll cycle and ABC transporters were strengthened in M chloroplasts, while carotenoid biosynthesis was strengthened and ABC transporters was weakened in BS chloroplasts. These showed that M and BS chloroplasts respond structurally and functionally to chilling stress.